Make srb2 a module

This makes it easier to add and remove srb2 from machines by simply
importing its module.

configuration.nix

@@ -11,6 +11,7 @@
     ./modules/dual-function-keys.nix
     ./modules/tlp.nix
     ./modules/osu
+    ./modules/srb2
   ];
 
   boot.loader.systemd-boot.enable = true;

modules/srb2/default.nix

@@ -0,0 +1,7 @@
+{ pkgs, ... }:
+
+{
+  environment.systemPackages = with pkgs; [
+    (pkgs.callPackage ./package {})
+  ];
+}